1. Field of the Invention
The present invention relates to structures of coils used in wireless AC power feeding/receiving.
2. Description of Related Art
A wireless power feeding technique of feeding power without a power cord is now attracting attention. The current wireless power feeding technique is roughly divided into three: (A) type utilizing electromagnetic induction (for short range); (B) type utilizing radio wave (for long range); and (C) type utilizing resonance phenomenon of magnetic field (for intermediate range).
The type (A) utilizing electromagnetic induction has generally been employed in familiar home appliances such as an electric shaver; however, it can be effective only in a short range of several centimeters. The type (B) utilizing radio wave is available in a long range; however, it cannot feed big electric power. The type (C) utilizing resonance phenomenon is a comparatively new technique and is of particular interest because of its high power transmission efficiency even in an intermediate range of about several meters. For example, a plan is being studied in which a receiving coil is buried in a lower portion of an EV (Electric Vehicle) so as to feed power from a feeding coil in the ground in a non-contact manner. The wireless configuration allows a completely insulated system to be achieved, which is especially effective for power feeding in the rain. Hereinafter, the type (C) is referred to as “magnetic field resonance type”.
The magnetic field resonance type is based on a theory published by Massachusetts Institute of Technology in 2006 (refer to Patent Document 1). In Patent Document 1, four coils are prepared. The four coils are referred to as “exciting coil”, “feeding coil”, “receiving coil”, and “loading coil” in the order starting from the feeding side. The exciting coil and feeding coil closely face each other for electromagnetic coupling. Similarly, the receiving coil and loading coil closely face each other for electromagnetic coupling. The distance (intermediate distance) between the feeding coil and receiving coil is larger than the distance between the exciting coil and feeding coil and distance between the receiving coil and loading coil. This system aims to feed power from the feeding coil to receiving coil.
When AC power is fed to the exciting coil, current also flows in the feeding coil according to the principle of electromagnetic induction. When the feeding coil generates a magnetic field to cause the feeding coil and receiving coil to magnetically resonate, large current flows in the receiving coil. At this time, current also flows in the loading coil according to the principle of electromagnetic induction, and power is taken out from a load connected in series to the loading coil. By utilizing the magnetic field resonance phenomenon, high power transmission efficiency can be achieved even if the feeding coil and receiving coil are largely spaced from each other.
[Patent Document 1] U.S. Pat. Appln. Publication No. 2008-0278264
[Patent Document 2] Jpn. Pat. Appln. Laid-Open Publication No. 2006-230032
[Patent Document 3] International Publication Pamphlet No. WO2006-022365
[Patent Document 4] U.S. Pat. Appln. Publication No. 2009-0072629
[Patent Document 5] U.S. Pat. Appln. Publication No. 2009-0015075
[Patent Document 6] Jpn. Pat. Appln. Laid-Open Publication No. 2008-172872
A receiving coil is connected to a capacitor to constitute an LC resonance circuit. A feeding coil is connected to another capacitor to constitute an LC resonance circuit. When the resonance frequencies of these LC resonance circuits coincide with each other, power transmission efficiency becomes maximum.
Since a frequency band not higher than 135 kHz is less constrained by Radio Act, there may be a case where it is preferable to set a resonance frequency at a low frequency band. In order to reduce the resonance frequency, it is only necessary to increase the inductance of a coil. To this end, it is only necessary to increase the number of windings of the coil. However, this increases the size of the coil. Further, when the coil is wound closely, the stray capacitance between coil conductors is increased, which poses a problem that resonance characteristics are degraded.
The present invention has been achieved based on the above issue, and a main object thereof is to increase the number of windings of a coil used in wireless power feeding of a magnetic field resonance type while suppressing the size of the coil.